Double conductor microelectronic structures are typically formed by growing a first insulating film over a single crystal silicon substrate, depositing a first layer of polycrystalline conductors (such as polycrystalline silicon) over the first insulating layer, placing a second insulating film over the structure and finally depositing a second layer of conductors over the second insulating film. The purpose of the second insulating film is to insulate the first and second conductive layers from each other. Such devices perform well only if both the first and second insulating films are good insulators. The first insulating film is usually a silicon dioxide layer grown over the single crystalline silicon substrate, and is well-known to be an excellent insulator. However, a significant problem in the art is that the second insulating film placed over the polycrystalline conductive layer is only a fair insulator, resulting in relatively high leakage currents between the two conductive layers through the second insulating film. While not subscribing to any particular theory, leakage of charge through the second insulating film is believed to be caused by surface spikes in the polycrystalline conductor surface. These surface spikes are believed to cause local enhancements in the electric field, resulting in high leakage currents through the second insulating film. Regardless of the theoretical explanation for this phenomenon, there remains the significant limitation of the art that insulating films placed over polycrystalline conductive layers do not provide insulation comparable to that afforded by oxide grown over single crystalline silicon. Because the current trend in microelectronic processing is to reduce the thickness of insulating films, this problem is particularly important because reduction in insulation film thickness causes an increase in leakage current for given operation voltages.
Double conductor microelectronic structures are used in various types of devices, for example, erasable programmable read only memories (EPROMS) which use the floating gate metal oxide semiconductor technology (FAMOS). FAMOS EPROMS are well known in the art and are discussed, for example, in Microelectronics, by the Editors of Scientific American, Freeman and Company, (1977) page 59, top figure. In this type of device, leakage currents through the second insulating film cause a loss of information or charge during and after writing of information into memory.
Charge coupled devices are also a species of double conductor devices. The inferior insulation qualities of oxide grown over polycrystalline silicon is also a limiting factor in the performance of charge coupled devices. In particular, charge coupled devices of the type discussed in Sequin, et al, Charge Coupled Devices, Academic Press (1975) may be formed of at least two layers of polycrystalline silicon electrodes which must be insulated from one another by an intermediate layer of oxide. Leakage of charge through the intermediate oxide layer between conductors which are connected to different clock signals will severely impair device performance.
Packing density of Very Large Scale Integration (VLSI) devices such as Dynamic Random Access Memories (RAMs) and Static RAMs is improved by utilizing double conductor microelectronic structures. Therefore, the problem of leakage current through the second insulating film in double conductor devices is significant in a Dynamic and a RAM Static RAM utilizing a double conductor structure.
One possible solution to the problem of inferior insulation afforded by insulation films placed over polycrystalline layers is to grow a silicon dioxide insulation film over polycrystalline silicon material at a very high temperature (1150.degree. C.). Although this high temperature technique is known to result in an insulating oxide film having better insulation qualtities, it still does not provide the excellent insulation afforded by oxide films grown over single crystal silicon. Furthermore, there are significant disadvantages associated with such a high temperature method. In order to achieve good insulation qualities in oxide films grown over polycrystalline material, the oxide must be grown at a high temperature of 1,150.degree. C., causing redistribution of implanted impurities in the substrate, such as, for example, source and drain diffusions of a Metal Oxide Semi-Conductor Field Effect Transistor (MOSFET). As a result, the area over which a MOSFET gate overlaps the diffusions may increase, increasing the parasitic capacitance between the diffusions and the MOSFET gate, which reduces the speed of the MOSFET. The redistribution of the source and drain diffusions also causes changes in the channel length between the source and drain diffusions leading to uncontrollable punch through currents.
A further disadvantage of high temperature deposition of oxide films over polycrystalline material is that the resulting device is more susceptible to change in electrical characteristics due to ionizing radiation damage from gamma rays and x-rays. As a result, devices having oxide layers grown at the high temperatures discussed above will not meet the radiation hardness requirements of military specifications.
In summary, the inferior insulation provided by oxide layers grown over polycrystalline silicon conductors is a problem limiting device performance which could be partially solved in the prior art only by using a high temperature oxidation process which degrades the device characteristics.